The present invention relates to an image forming method and a printer, copier, facsimile apparatus or similar image forming apparatus using the same. More particularly, the present invention relates to an image forming apparatus of the type developing a latent image formed on a photoconductive drum or similar image carrier with toner or similar developer stored in a developing unit and transferring the resulting toner image to a paper sheet or similar recording medium.
An image forming apparatus of the type described is operable with a magnetic or nonmagnetic single ingredient type developer for developing a latent image electrostatically formed on an image carrier. A developing system using this type of developer includes a developer carrier contacting the image carrier while carrying the developer thereon in the form of a thin layer. The developer carrier is implemented as an elastic developing roller. With such a developing system, it is possible to obviate the scattering of the developer and thereby enhance the reproducibility of dots by, e.g., increasing a contact pressure between the image carrier and the developer carrier and a linear velocity ratio of the developer carrier to the image carrier.
However, the above developing system is susceptible to the fine oscillation of the image carrier and developer carrier. For example, when the contact pressure is increased, a drive source for setting the contact pressure must bear a heavy load. This is likely to bring about defective images ascribable to banding caused by irregular rotation.
To prevent the load on the drive source from increasing, the contact pressure of the developer carrier acting on the image carrier may be reduced, as proposed in the past. This conventional scheme, however, has a problem that the contact pressure is lower at the axially intermediate portion of the developer carrier than at the axially opposite end portions, causing the intermediate portion of an image to be lost. In light of this, Japanese Patent Laid-Open Publication No. 10-20857, for example, teaches a developer carrier having an intermediate portion greater in diameter than opposite and portions.
The developer carrier is generally formed of rubber. However, it is difficult to achieve dimensional accuracy with rubber, as distinguished from metal. This is particularly true when rubber has low hardness. In this condition, should the contact pressure between the image carrier and the developer carrier be lowered to reduce the load on the drive source, contact between them would easily become non-uniform and render irregularities conspicuous in an image. It is therefore necessary to strictly control the dimensional accuracy of the developer carrier. The dimensional accuracy involves various parameters, e.g., a tolerance in outside diameter, oscillation and cylindricality. However, despite the control over such parameters, it is sometimes difficult to reduce irregular images due to conditions in which the developer carrier is mounted. In addition, strict control over the dimensional accuracy increases cost.
The developer layer, or toner layer, deposited on the developer carrier contacts the surface of the image carrier. In this condition, an electrostatic force exerted by a latent image formed on the image carrier attracts toner existing in the toner layer and causes it to deposit on the latent image. In practice, however, a non-electrostatic force derived from e.g., the contact condition between the toner layer and the image carrier often causes the toner to deposit on the image carrier, particularly the background thereof where the electrostatic force of the latent image does not act on the toner. This contaminates the background to a noticeable degree.
Japanese Patent Laid-Open Publication No. 8-254933, for example, discloses an arrangement for obviating the background contamination of the image carrier. The arrangement is such that toner density outside the image area of the image carrier is sensed to determine the degree of background contamination. A device for applying a lubricant is held in contact with the image carrier. The amount of the lubricant to be applied to the image carrier is controlled in terms of a contact pressure in accordance with the degree of background contamination, thereby enhancing a cleaning effect.
Japanese Patent Publication No. 7-117788, for example, proposes a developing device operable with a nonmagnetic single ingredient type developer. In the developing device taught in this document, the amount of developer to deposit on the developer carrier is selected to be 0.6 mg/cm2 to 1.2 mg/cm2. In addition, the ratio of the moving speed V1 of the developer carrier to the moving speed V2 of the image carrier is confined in the range of 0.6xe2x89xa6V1/V2xe2x89xa60.9. Those specific conditions are used to mechanically remove developer particles adhering to each other with a weak force without increasing the drive force.
The above lubricant scheme and linear speed ratio scheme, however, have a drawback that because the image carrier and developer carrier contact each other via the toner layer, the toner is apt to deposit on the image carrier due not only to the electrostatic force but also to the non-electrostatic force. This aggravates the background contamination of the image carrier.
To solve the above problem, it is a common practice to increase the contact pressure and linear velocity ratio at the time when image forming process conditions are set. The contact pressure, however, causes the previously stated banding to occur when increased. Also, the linear speed ratio of the developer carrier to the image carrier is apt to cause the toner to concentrate at the trailing edge of an image when increased.
On the other hand, the toner deposits on the developer carrier in a plurality of layers each having a particular amount of charge. Part of the toner existing in upper layers and short of charge are scattered onto the image carrier, contaminating the background of the image carrier. Further, as the toner coheres due to aging, an adhering force acting between toner particles increases and makes it difficult for them to migrate toward the image carrier when a halftone image, for example, is to be formed. This prevents a halftone image from being faithfully reproduced. Moreover, a defective image ascribable to banding becomes more conspicuous as the toner layer becomes thicker. Specifically, although a regulating member causes the toner to form a thin layer, any change in the contact pressure of the regulating member ascribable to irregular rotation makes the toner layer thickness irregular. Consequently, among toner particles existing on the developer carrier, the particles in upper layers not firmly deposited despite the electrostatic and non-electrostatic forces are susceptible to a change in thickness, causing the thickness of the toner layer to vary.
Japanese Patent Laid-Open Publication No. 9-197713, for example, teaches a method for obviating the fall of image density and tonality, paying attention to the fact that the cohesion of toner, among others, effects the reproducibility of a halftone image, as stated above. The method consists in specifying the variation of a degree of cohesion, an angle of repose and a loose apparent specific gravity due to aging as well as the configuration of toner particles. Also, Japanese Patent Laid-Open Publication No. 9-73229 proposes to specify the bulk density of the thin toner layer and the amount of toner to deposit on the image carrier for preventing an image from being blurred.
However, the above Laid-Open Publication No. 9-197713 contemplates to prevent image density from falling and therefore assumes a condition wherein the maximum amount of toner to deposits. The above Laid-Open Publication No. 9-73229 contemplates to increase image density by preventing the toner from depositing on a non-image area and, for this purpose, makes the thickness of the thin toner layer uniform. None of such schemes is therefore directed toward a halftone image. More specifically, the image carrier and developer carrier sandwich the toner at a nip for development and cause it to easily cohere. Cohesion increases the packing density of the toner and therefore the adhering force acting between the toner particles, degrading the reproducibility of a halftone image.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 4-372981, 6-258933, 6-295130, 8-305075, 9-179389, 10-69162, 11-84878 and 11-149174 as wall as in Japanese Patent No. 2,715,337.
It is therefore a first object of the present invention to provide an image forming apparatus capable of reducing irregular images without increasing cost by including a developer carrier contacting an image carrier and provided with dimensional accuracy based on new criteria.
It is another object of the present invention to provide an image forming apparatus of the type including a developer carrier for electrostatically retaining a developer thereon and a regulating member for causing the developer to form a thin layer, and capable of protecting the image carrier from background contamination and insuring faithful reproduction of a halftone image.
In accordance with the present invention, in an image forming method for electrostatically forming a latent image on the uniformly charged surface of an image carrier by optical writing, causing a developer carrier on which a single ingredient type developer is deposited to contact the image carrier to thereby develop it while controlling a contact pressure, transferring the developed image to a recording medium, and fixing the developed image on the recording medium, the developer carrier is implemented as a cylindrical member. Assume that any two points on ridgelines are connected by a line in any section passing through the axis of the developer carrier, that the maximum distance between ridgelines present between the above line and the two points is T in the normal direction, and that the displacement of the developer carrier to occur when a contact pressure is applied to the developer carrier located at one side between the two points and contacting facing one of the ridgelines with no load is L. Then, there holds a relation:
T less than L
Also, in accordance with the present invention, in an image forming apparatus for electrostatically forming a latent image on the surface of an image carrier having a coefficient of friction xcexc lying in a range of 0.1xe2x89xa6xcexcxe2x89xa60.4 and uniformly charged beforehand by optical writing, causing a developer carrier on which a single ingredient type developer is deposited to contact the image carrier to thereby develop the latent image while controlling a contact pressure, transferring the developed image to a recording medium, and fixing the developed image on the recording medium, the volume mean particle size Tr (cm) of the developer, the amount M (mg/cm2) of the developer to deposit on the developer carrier and the bulk density xcfx81 (mg/cm3) of the developer have a following relation when the developer is pressed by a pressure of 100 gf/cm2:
0.5 xcfx81Rtxe2x89xa6Mxe2x89xa61.2 xcfx81Rt
The developer should preferably have a loose apparent density of 0.35 or above. Also, the developer should probably have its configuration determined by sphericality and have sphericality of 90% or above with respect to true sphericality. Further, a plurality of particles should stay in the vicinity of the developer carrier.